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Fracture-Correlated Lineaments at Great Bay, Southeastern New Hampshire
Open-File Report 02-13


BEDROCK AND FRACTURES

The Kittery Formation, where exposed along the bay, consists of metasandstone with 0.05- to 0.20-m-thick beds. The Eliot Formation, where exposed along the bay, is composed of gray to green interbedded phyllite and fine-grained sandy, calcareous phyllite in 0.01- to 0.02-m, thinly laminated beds. The Exeter Diorite includes diorite and gabbro, with some granodiorite and granite that is light-gray to black in color, and fine- to coarse-grained in texture (Novotny, 1968; Lyons and others, 1997). Mesozioc diabase dikes are seen in outcrops along the shore of the bay and ranged from 0.2 to 30 m thick, trending primarily northeast.

The bedding in the Kittery Formation has a peak trend of 106° (fig. 3a). The bedding and dominant foliation trend of the Eliot Formation is 26º (fig. 3b). A regional overturned anticline is mapped near the contact between the Kittery and Eliot Formations, near the middle of the bay (Lyons and others, 1997).

figure 3
Figure 3. Contoured stereo-net plots and azimuth-frequency plots showing bedding and foliation data for (A) the Kittery Formation and (B) the Eliot Formation at Great Bay, N.H. N equals the number of bedding and foliation measurements represented. The length of the family peaks on the azimuth-frequency plots indicates the normalized height. Principal peaks with normalized concentrations greater than 50 percent are labeled. Circle interval equals 20 percent, increasing outward.

The metasedimentary Kittery and Eliot Formations are part of a green schist regional facies with chlorite and biotite phases. Metamorphism occurred between the Devonian and Permian (Lyons and others, 1997). The youngest deformational event, possibly the sixth, is evident as north-south trending fractures in the Exeter Diorite and the Kittery Formation (Fargo and Bothner, 1995). This trend is reflected in the maximum peak trend in all of the fracture data (fig. 4a), the Exeter Diorite and Kittery Formation (figs. 4b, c) respectively, and the peak trend of the smallest fracture spacing measured (fig. 5a).

fig4
Figure 4. Contoured stereo-net plots and azimuth-frequency plots showing fracture data for (A) all fractures, (B) Exeter Diorite, (C) Kittery Formation, (D) Eliot Formation, (E) diabase dikes and sills, and (F) open and vug-filled fractures at Great Bay, N.H. N equals the number of fracture data measurements represented. The length of the family peaks on the azimuth-frequency plots indicates the normalized height. Principal peaks with normalized concentrations greater than 50 percent are labeled. Circle interval equals 20 percent, increasing outward.

fig5
Figure 5. Contoured stereo-net plots and azimuth-frequency plots showing fracture data binned by spacing for (A) 1- to 4-centimeter spacing, (B) 5- to 10-centimeter spacing, (C) 11- to 40-centimeter spacing, and (D) 41- to 200-centimeter spacing at Great Bay, N.H. N equals the number of fracture data measurements represented. The length of the family peaks on the azimuth-frequency plots indicates the normalized height. Principal peaks with normalized concentrations greater than 50 percent are labeled. Circle interval equals 20 percent, increasing outward.

The Exeter Diorite fracture family has a maximum fracture-family-peak trend of 6° (fig. 4b). The Eliot Formation's largest fracture family trends 144° (fig. 4d). Diabase dikes and sills and parallel fracture sets strike northeast in three peak trends--29°, 51°, and 71° (fig. 4e).

Outcrops of Kittery and Eliot Formations have approximately six fracture orientations in each outcrop, most having little apparent aperture. Four to seven fracture sets, each with uniform fracture spacing and orientation, are common in most outcrops along with up to eight isolated fractures with different orientations. Locally, intersecting fractures produce shattered cliff faces and small talus piles, which are found for example at the outcrops at the mouth of Crommet Creek. Diabase dikes and sills have northeast strikes, with parallel, nearly vertical fractures (fig. 4e).

In the Kittery Formation, the two largest fracture families trend 89° and 171° (fig. 4c). The 89° trend correlates with a peak of the open and vug-filled fractures (86°) (fig. 4f). Bed-parallel fracturing in the Kittery Formation locally (such as at the hinges of outcrop-scale folds) is silicified, vuggy, or open, with apertures as great as 1 cm in width (figs. 3a, 4f). Fractures were grouped into bins based on spacing measurements; this grouping shows peak trends differ with spacing (fig. 5). Open- and vug-filled fracture
peaks also have the same trend (86° and 98°) as the two largest east-west-trending fracture peaks in the widest-spaced-fracture data (fig. 5d).


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